A magnetic random access memory (MRAM) is a type of a resistance change memory. As techniques for writing data to the MRAM, magnetic field writing and spin-transfer torque writing have been known. Among these techniques, the spin-transfer torque writing has advantages in higher integration, lower power consumption, and higher performance because of the property of a spin-transfer torque device that a smaller amount of a spin injection current is necessary for magnetization reversal as the size of magnetic bodies becomes smaller.
A spin-transfer torque MTJ (Magnetic Tunnel Junction) element has a stacked structure in which a nonmagnetic barrier layer (an insulating thin film) is sandwiched between two ferromagnetic layers, and stores data by a change in a magnetic resistance caused by spin-polarized tunneling. The MTJ element can be switched into a low resistance state or a high resistance state depending on the magnetization orientations of the two ferromagnetic layers. The MTJ element is in a low resistance state when the magnetization orientations (spin directions) of the two ferromagnetic layers are in a parallel state (a P state), and in a high resistance state when the magnetization orientations (spin directions) thereof are in an anti parallel state (an AP state).
It is desired to downscale the MRAM like a DRAM. If the MTJ elements are not arranged equidistantly, that is, if the MTJ elements are not arranged uniformly in a plan layout, variations in the shape and size of the MTJ element occur among memory cells. The variations in the shape and size of the MTJ element lead to a variation in a signal read from each memory cell. Furthermore, if the MTJ elements are not arranged uniformly, the MTJ elements are adjacent to one another at different distances. As a result, if the distances between the MTJ elements are to be reduced for downscaling purposes, it is disadvantageously difficult to process the MTJ elements.